1. Field of Invention
The invention relates to wafers and methods of preparation thereof and, in particular, to wafers comprising spinel crystal and having low warp and bow characteristics.
2. Discussion of Related Art
Optoelectronic devices can be fabricated by depositing semiconductor material on substrates or wafers. Preferably, these substrates have a compatible crystal structure that promotes growth of the deposited semiconductor into a desirable, as-deposited lattice structure. Substrates which can be utilized can be comprised of sapphire or spinel. To provide desirable optoelectronic device performance, the surface quality of these substrates has been studied and improved.
For example, Robinson et al., in U.S. Pat. No. 3,808,065, disclose a method of polishing sapphire and spinel. Spinel wafers are polished by immersing the wafers in a melt of borax at a temperature of 1000° C. to 1200° C.
Berkenbilt et al., in U.S. Pat. No. 3,964,942, disclose polishing single crystal dielectrics, such as sapphire and magnesium spinel. Polishing involves immersion in a mixture of sulphuric and phosphoric acid at a temperature of 200° C. to 325° C.
Laconto, Sr. et al., in U.S. Pat. No. 5,989,301, disclose optical polishing formulations.
Garg et al., in U.S. Pat. Nos. 6,238,450, 6,258,137, and 6,364,920, disclose ceria powders, CMP products, and CMP formulations.
Robinson et al., in “The Chemical Polishing of Sapphire and Spinel,” RCA Rev., vol. 34, pp. 616-629, December 1973, disclose etching of as-sawed, rough-lapped sapphire substrate wafers in a Al2O3/borax melt.
Yanina and Carter, in “Moving Steps and Crystal Defect on Spinel Surfaces,” Mat. Res. Soc. Symp., vol. 620, pp M9.4.1-6, 2000, disclose the morphology of reconstructed MgAl2O4 spinel and heat treated polished MgAl2O4 spinel at a temperature of 1200° C. to 1600° C.
Wang, in “Growth and Characterization of Spinel Single Crystals for Substrate Use in Integrated Electronics,” J. Appl. Phys., vol. 40, no. 9, pp. 3433-3444, August 1969, disclose spinel wafers prepared by mechanical lapping and hydrogen annealing or etching by H2SO4, H3PO4, KOH, B2O3, V2O5, Na2B4O7, and PbF2.
Aeschlimann et al., in “The Chemical Polishing of Magnesium Aluminate Spinel in Phosphoric Acid,” Mat. Res. Bull., vol. 5, pp. 167-172, 1970, disclose chemically polishing spinel in phosphoric acid at 390° C. in twenty minutes and note that polishing times greater than thirty minutes results in lost surface flatness.
Dwilcusuma et al., in “Study on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatments,” J. of Electrochem. Soc., vol. 149, issue 11, pp. G603-G608, November 2002, disclose etching sapphire substrates using sulfuric acid, phosphoric acid, and a mixture of sulfuric and phosphoric acids at a temperature of 100° C. to 300° C.
Korube et al., in “Magnetic Field-Assisted Lapping,” Bull. Japan Soc. Of Prec. Engg., vol. 20, no. 1, pp. 49-51 March 1986, disclose a lapping method utilizing a magnetic fluid.
Reisman et al, in “The Chemical Polishing of Sapphire and MgAl Spinel,” J. Electrochem Soc., vol. 118, no. 10, pp. 1653-1657 October 1971, disclose air annealing studies of sapphire and spinel.